Achieved 2.56Tbps transfers over the air using light split into four beams.

The limit of communication using light isn't its speed, but the number of ways of encoding and decoding information using photons. Today's fiber optic communications rely on light of different wavelengths to carry data from multiple sources carried along the same cable: each distinct signal is carried at a separate wavelength, which are then separated at the receiving end. A second method uses the polarization of light, which relies on two distinct ways photons can spin around the direction of motion. Sending two signals that differ only by polarization doubles the rate of data transfer.

A third method involves manipulating the shape of the photons themselves: twisting the light's wavefront into a ring, so that the photons rotate rather than merely spin around the direction of motion. The size of the ring is independent both of the wavelength of the light and the polarization. In principle at least, it has no limit to the number of possible configurations. Researchers Jian Wang et al. achieved terabit (Tbit/s) data transfer rates over open-air distances of about a meter, through combining four beams of different rotations (channels). They demonstrated both the generation and receiving of distinct data from the different channels (multiplexing and demultiplexing), using a laser beam of a single wavelength to generate all of them. While this result is not currently usable in fiber optics, it's a significant step toward exploiting a thus-far unused property of light to increase the rate of telecommunication.

Electrons in atoms have three distinct quantum properties: their energy level, their orbital angular momentum, and their spin angular momentum. Angular momentum in the macroscopic world can be thought of as how difficult it is to stop a spinning object. Microscopically, it dictates the kinds of transitions that are possible between different quantum states within the atom. This gives rise to the unique spectrum different elements and molecules possess. Orbital angular momentum, as the name suggests, depends on the orbit of the electron, but spin is a property of the electron alone.

Similarly, photons have distinct quantum properties, though they cannot be trapped in the same way as electrons. They possess energy (determined by their wavelength), polarization (which is light's version of spin), and orbital angular momentum (OAM). Photon OAM is a rotation of the photon around its direction of motion; think of it as traveling in a helical pattern, akin to a spiral staircase. The amount of OAM is independent of polarization and energy, and—unlike polarization, which has only two possible states—theoretically can have an infinite number of possible values. Additionally, by combining two or more OAM states in a single photon, many possible rotation configurations can be created (including rather cool spiral patterns, which the authors showed in their paper).

Light can carry information in the form of orbital angular momentum (OAM), where the photons twist in a spiral pattern. That opens up a large number of new channels for data transfer.

Alan Wilner et al.

The researchers started with laser light of a specific wavelength, then split it into four beams, each carrying four different bits of information. Each beam was reflected off a different phase mask, a special liquid crystal whose structure produces the helical wavefront. The particular filters made four concentric rings of wavefronts. Each of these beams was then passed through a polarizing beamsplitter (PBS), resulting finally in eight separate channels (beams of light carrying a unique set of bits). These were then combined and sent across a distance of about one meter to a receiver.

At the receiving end, the total beam was passed through polarizing filters to select each of the two polarizations. The resulting light was reflected from a second set of phase masks; a match between the particular helical wave front and the phase mask changed the light from a circular pattern back to the original unshaped wave. They achieved as high as 2.56Tbit/s data transfer.

Additionally, the researchers exchanged data between beams by reflecting both off a phase mask unmatched to either. The reflection swapped the OAM values between the beams. This allows for flexible data transfer and processing, possibly for quantum computing applications.

While this was an excellent proof-of-principle experiment, it will be a while before we'll see OAM-based telecommunication. For starters (as both the authors and accompanying commentary point out), optical communication through air is prone to many problems as the light scatters off air molecules. (However, communication through vacuum, as between spacecraft, is still a possibility, at least over distances where dispersion isn't a major issue). One meter is an insignificant distance for most communication purposes, so optical signals are generally passed through fiber optic cables. However, standard fiber optic cables are not capable of carrying this kind of multi-channel signal, while fibers that can handle it experience problems of cross-contamination between channels when carrying data under high bit-rates.

Nevertheless, opening up the OAM degree of freedom available in photons will possibly lead to significant increases in data transfer, since technical challenges often lead to novel technical solutions. While the "infinite" number of available OAM channels doesn't necessarily even mean thousands in practice, even a modest increase over the existing transfer rate that can be carried by a single frequency of light is noteworthy.

I wonder if the hardware is small enough, could this be a viable solution to some of the high speed transfer goals of putting optical communication in PCs to replace copper? Say...high speed links between RAM and the CPU for example? (if that were the case, I really hope they call it a vacuum tube!)

Agreed very cool experiment, and great summary of it too. Have difficulty wrapping my head around some of these waveform + photon manipulations, but the analogies & diagram help me understand how it works a bit more (even if the 'why' of the physics is a bit beyond my reach)

Interesting read. Pass this on to the materials engineers and see if they can make fibers capable of carrying it reasonable distances and we might have something here in 10 years...

Multimode fibers can transport multiple spatial modes, but they do so at the expense of massive dispersion, so you can't use wide bandwidth down them over any real distance. So you'd trade a factor of a 100 in temporal bandwidth to gain a factor of 10 in spatial bandwidth and get a free factor of 10 in complexity along with it

Good to see a discussion of this that includes the downsides, many places on the internet are skipping that.

Yes this is an annoyance of mine as well. You rarely see mentioned that there is zero interest or use for ways to transmit 100000 Tbit/s down an optical fiber that aren't tremendously cheaper then WDM or the couple cents a foot that using two fibers costs. Thats what is usually missed, fiber has basically unlimited bandwidth already, its just so damn expensive that we barely use any of it. Unless your super new awesome method is cheaper per bit its not really all that useful no matter how big of a number you can get for the bandwidth.

BobTreehugger wrote:

I guess the goal is to use radio waves for this eventually, to make super-wifi or whatever but translating this to not use a laser is probably the bigger issue.

Yeah probably. This is actually pretty similar to the 2x2 stuff they do for the new >1gigabit 802.11 standards, just with phase modulators rather then phased arrays since you can't build those at optical frequency.

Interesting read. Pass this on to the materials engineers and see if they can make fibers capable of carrying it reasonable distances and we might have something here in 10 years...

Multimode fibers can transport multiple spatial modes, but they do so at the expense of massive dispersion, so you can't use wide bandwidth down them over any real distance. So you'd trade a factor of a 100 in temporal bandwidth to gain a factor of 10 in spatial bandwidth and get a free factor of 10 in complexity along with it

You are right. But it is a materials problem in a way. Because Photonic Crystal fibers can actually cut down on those dispersion losses. The problem with PCFs is still they are nowhere near to telecom fiber loss levels still.For many years people scratched their heads and wandered why, but recent papers have shown almost all of that loss comes from scattering losses due to strains frozen in the glass during fiber pulling. If materials engineers can make those strains go away, the problems you mention would cease to exist.

You are right. But it is a materials problem in a way. Because Photonic Crystal fibers can actually cut down on those dispersion losses.

I think you misunderstand differences between material dispersion (e.g. frequency dependent speed of light) and modal dispersion (different modes having different propagation velocity independent of the refractive index of the material). PCFs can be made to have no material dispersion, but modal dispersion is inherent in the waveguide. No amount of engineering can eliminate it because it arises from the need to have different velocities from distinct modes.

In situations where modes have equal propagation velocity (and hence no modal dispersion), you have massive cross talk between because modes are phased matched. Thats why in PM fibers they deliberately add modal dispersion so that each polarization doesn't phase match and thus randomize.

So switching to a PCF doesn't get you anything in this case, although obviously its really nice for WDM.

I wonder if the hardware is small enough, could this be a viable solution to some of the high speed transfer goals of putting optical communication in PCs to replace copper? Say...high speed links between RAM and the CPU for example? (if that were the case, I really hope they call it a vacuum tube!)

Agreed very cool experiment, and great summary of it too. Have difficulty wrapping my head around some of these waveform + photon manipulations, but the analogies & diagram help me understand how it works a bit more (even if the 'why' of the physics is a bit beyond my reach)

I thought Intel and the like were exploring fiber-optic computing.

This seems more for large-scale transfers. But, looking at other peoples comments, and reading the downsides in the articles, it's once again a nice proof-of-concept with quite a few hurdles in place before something useful or practical comes about.

2.56Tbps *is* awesome, and I am in no way out to minimize the importance of this research, but that bandwidth from one spot to another is quite a different thing than 100,000 25.6Mbps streams all getting from 100,000 points to 100,000 other points with reasonable latency and without consuming as much power as a town.

As they say, never underestimate the bandwidth of a minivan full of DVD's.

Wait, can't light be polarized to any number of angles? It's just that if you send more than 2 angles through at once, it's impossible to decode it into its 3 or more individual parts?

You're basically right. You can set the photon to be any value of polarization you want, but the detection has to be a choice between just two orthogonal values, like horizontal/vertical, or clockwise/counter-clockwise. https://en.wikipedia.org/wiki/Elliptical_polarization

Because a telcom fiber only carries a single mode with a Gaussian field profile. Angular momentum is not an accessible degree of freedom in that configuration.

You can make multimode fibers that have all sorts of degrees of freedom (although i'm not sure how you would make one that maintained angular momentum), but they have either very limited length, or very limited bandwidth. So basically, you're SOL.